1. Field of the Invention
The present invention relates to an art for compensating for a threshold voltage of a transistor employed in a semiconductor circuit. More particularly, this invention is concerned with a circuit effective in compensating for a difference of a threshold voltage from another and the configuration of a sense amplifier using the circuit.
In recent years, a short channel effect, narrow channel effect, or the like has come to light with the advent of a finely-structured semiconductor device. The short channel effect is a phenomenon that a shorter gate results in a lower threshold voltage, while the narrow channel effect is a phenomenon that a narrower gate results in a higher threshold voltage. If the length or width of a gate differs from one another due to a difference of a transistor from another occurring in the course of manufacturing, the threshold voltage of the transistor becomes different from another. Moreover, reportedly, an avoidable difference of a threshold voltage from another results from the natural distribution of ions occurring during ion implantation performed for forming channels. If the structure of a semiconductor device becomes finer in the future, it would be hard to suppress such a difference. An art of circuit design that takes account of the difference is demanded.
2. Description of the Related Art
In the past, a method of compensating for a threshold voltage has been devised as one of design arts taking account of a difference of a threshold voltage from another. A so-called well drive approach is well-known as one of the methods. The well drive approach is an approach in which a threshold voltage is practically changed by changing the potential at a well in a semiconductor substrate. This approach is effective when, for example, the threshold voltage of a chip is the same over the whole chip or over a relatively wide area in the chip, but there is a difference in threshold voltage between lots of chips, and a system does not operate under intended specifications because of the difference. However, if the threshold voltage of a certain area of a chip is rather high but that of another area thereof is low, the operating speed of the chip differs from area to area. In the whole chip, delays occur to result in a clock skew. Consequently, there arises a drawback that a circuit does not operate normally.
Thus, the known method is effective when there is a difference in threshold voltage between chips or there is a difference in threshold voltage between relatively wide areas of a chip. However, the difference in threshold voltage is not limited to these examples. Such a situation that, for example, a difference in threshold voltage between adjoining transistors cannot be ignored will presumably arise more frequently in the future. This is because the situation is, as mentioned above, unavoidable in the process of manufacturing.
A circuit that may be greatly affected by a difference in threshold voltage between adjoining transistors is a flip-flop type sense amplifier. Several types of sense amplifiers for DRAMs have been proposed as a means for nullifying the adverse effect.
For example, one type of sense amplifier is provided with a facility for compensating for the threshold voltages of transistors constituting a flip-flop during a sensing operation. However, this type of sense amplifier must carry out an additional operation such as threshold voltage compensation in addition to the sensing operation, and is therefore unsuitable for high-speed operations that have been demanded more often in recent years.
Another type of sense amplifier is such that the source and drain of a transistor are changed between a sensing operation and threshold voltage compensation (connections are changed). However, if the threshold voltage of the transistor has a difference dependent on a direction, this type of sense amplifier cannot compensate for the difference successfully.
A pre-charge method has widely been adopted as a method for reading cell data from a DRAM. In some known approaches, a supply voltage Vcc has been used for pre-charge. However, since a sense amplifier has peripheral circuits thereof formed with CMOSs nowadays, a Vcc/2 pre-charge approach has become mainstream. For this reason, sense amplifiers adopting the Vcc pre-charge approach can preferably cope with the Vcc/2 pre-charge reading.